In a V-block refractometer which is one example of refractometers, a sample is placed on a V-shaped groove formed on a V-block prism, and the sample is irradiated with measurement light through the V-block prism. The measurement light passing through the sample is detected by a detector, whereby a refractive index of the sample can be measured (see Patent Document 1 below, for example).
With the refractometer of this type, a part of measurement light passing through a sample to be guided to a detector is split by a beam splitter, for example, and guided to an eyepiece unit (not illustrated). When performing a work for adjusting the position of the V-block prism, or the like, an operator can perform the work, while visually recognizing the state of the measurement light, by seeing the inside of the refractometer from the eyepiece unit.